1. Field of the Invention
This invention relates generally to a nickel, chromium, iron welding alloy, articles made therefrom for use in producing weldments, and weldments and methods for producing these weldments. The invention further relates to welding electrode, welding wire and flux chemical coating compositions and welding methods to obtain weld deposit compositions that are particularly resistant to ductility dip cracking, as well as being resistant to primary water stress corrosion cracking in a nuclear power generation environment.
2. Description of Related Art
In various welding applications, including equipment used in nuclear power generation, weldments are required that provide resistance to various cracking phenomenon. This includes not only stress corrosion cracking but hot cracking, cold cracking, and root cracking as well.
Commercial and military nuclear power generation has existed since the second half of the 20th century. During this time, the industry has replaced the first generation of NiCrFe alloys having 14-15% chromium with alloys having higher chromium contents on the order of 30%. This change was predicated on the discovery that stress corrosion cracking in nuclear pure water could be avoided with alloys of this type that contained chromium in this amount. NiCrFe alloys having Cr contents on the order of 30% have now been in use for about 20-25 years.
The specific application for nuclear power generation equipment that requires the majority of welding and welded products within the nuclear power plant is the fabrication of the nuclear steam generator. This equipment is essentially a large tube and shell heat exchanger that generates steam from secondary water from primary nuclear reactor coolant. The key component of this steam generator is the tubesheet. The tubesheet is sometimes 15-20 feet in diameter and well over one foot thick and is usually forged from a high strength low alloy steel that must be weld overlaid with a NiCrFe alloy that has good fabricability and is resistant to stress corrosion cracking in nuclear pure water. Due to the size of the tubesheet, the weld deposit sustains substantial residual stress during overlay. Furthermore, the weld metal overlay must be capable of being rewelded after being drilled to provide openings therein to receive thousands of small steam generator tubes. These tubes must be seal-welded to the overlay weld deposit to make helium-leak-tight welds. These welds must be of extraordinary high quality and must provide 30-50 year life with high predictability. In addition, in both the overlay weld deposit and the welded steam generator tubes, excellent crack resistance must be provided. This requirement, with respect to resistance to hot cracking, also termed “solidification cracking,” and stress corrosion cracking has been met by most of the existing 30% chromium weldments.
In addition to hot cracking resistance and stress corrosion cracking resistance, the tube-to-tubesheet welds require root cracking resistance. The tube-to-tubesheet weld is made by melting the tube end together with a ring of the weld overlay material surrounding the tube (with or without the use of additional filler metal) to thereby seal the space between the tube wall and the opening in the tubesheet. There is a tendency for these welds to crack at the bottom of the weld at the interface of the tube to the tubesheet. This type of cracking is referred to as “root cracking” because it occurs at the root of the weld. The existing 30% chromium welding alloys are not resistant to root cracking.
A third type of cracking that may be encountered is cold cracking, also known as “ductility dip cracking” or “DDC”. This cracking only occurs in the solidified state after weld solidification has been completed. After solidification occurs, shrinkage stresses begin to develop as a result of the reduction in volume of the welding alloy at lower temperature. At the same time, once solidification is complete, ductility recovery occurs rapidly for a few hundred degrees, followed by a sharp temporary loss in ductility, and again followed by a more gradual continuous recovery of ductility until ambient temperature is reached. If the residual stress of cool-down is sufficiently large when the alloy exhibits this sharp ductility loss, solid state cracking (DDC) may occur. This results from portions of the microstructure not having sufficient strength or ductility to resist the stress at the prevailing temperature. The commercially available 30% chromium welding alloys presently available are not sufficiently resistant to DDC.
Ductility Dip Cracking (DDC)/cold cracking has become a topic of interest over the past ten years in the fully austenitic nickel-chromium-iron alloys and welding products used in the nuclear industry. The scientific community has learned that NiCrFe alloys with approximately 30% Cr exhibit resistance to primary water stress corrosion cracking (PWSCC) in the nuclear environment. However, the higher Cr levels, coupled with reduced Nb, tend to give weld deposits that solidify epitaxially with long straight dendrite boundaries. These boundaries, when subjected to high strain and elevated temperatures, are particularly susceptible to DDC. The phenomenon seems to be more prevalent in the 30% Cr-containing nickel alloys such as Inconel alloy 690 and welding products of the AWS class NiCrFe-7. The tendency for DDC cracking has been addressed by the invention of Inconel Filler Metal 52M and Weld Strip 52M (AWS class NiCrFe-7A-UNS3N06054). These products are covered by the present inventor's invention disclosed in U.S. Pat. No. 6,242,113, the contents of which are incorporated by reference herein in their entirety. The solution for DDC/cold cracking in flux coated electrodes is specifically addressed by the present application.
Also of interest as background information are the various specifications of the American Welding Society (“AWS”) and American National Standards Institute (“ANSI”), namely, ANSI/AWS Specification A5.11/A5.11M:2005, entitled “Specification for Nickel and Nickel-Alloy Welding Electrodes for Shielded Metal Arc Welding” and ANSI/AWS Specification A5.14/A5.14M:2005, entitled “Specification for Nickel and Nickel-Alloy Bare Welding Electrodes and Rods”. Both of these specifications are incorporated by reference in their entireties herein.